This invention relates in general to defect detection, and, in particular, to an improved system for detecting anomalies on surfaces, such as particles and surface-originated defects such as crystal-originated particles (“COPs”) and other defects.
The SP1TBI™ detection system available from KLA-Tencor Corporation of San Jose, Calif., the Assignee of the present application, is particularly useful for detecting defects on unpatterned semiconductor wafers. The SP1TBI system provides the capability to review areas of the surface where potential anomalies have been identified, in a mode known as microview. The SP1TBI system determines the presence of potential anomalies by comparing intensities of detector outputs with a threshold. If the intensity of radiation detected by the detector exceeds the threshold, a potential anomaly is then determined to be present at the location from which the scattered radiation is detected. The intensity value is then stored and identified to be indicative of a potential anomaly at a corresponding location of the surface.
After the entire surface (such as the surface of a semiconductor wafer, reticle or display panel) has been inspected in this manner, the areas containing locations where the potential anomalies are found are then re-examined to determine whether anomalies are actually present at such locations. In this determination, it is useful and often necessary to compare the radiation scattered by the surface at the locations determined to have potential anomalies to radiation scattered by portions of the surface surrounding such locations, such as portions within patches or areas of the surface, each patch or area containing one of such locations. In the microview mode, the SP1TBI system re-scans the portions of the surface surrounding the locations determined to have potential anomalies, and records the detector output intensities so obtained. This is necessary because the scattered radiation intensities detected by the detectors from areas surrounding the potential anomalies (and where no potential anomaly has been detected) fall below the threshold and are therefore discarded and not stored. A comparison of the detector output intensities of radiation scattered by the surface at the locations determined to have potential anomalies to the detector output intensities of radiation scattered by portions of the surface surrounding such locations will confirm whether anomalies are indeed present at such locations. These confirmed locations with potential anomalies can then be examined in greater detail or at higher resolution. The microview mode is useful since there may be a large number of locations with potential anomalies, and the re-scanning and comparison process in this mode may reduce the number of locations that would need to be examined in greater detail or at higher resolution.
While the above-described microview mode of the SP1TBI system is useful, it requires re-scanning of the surface. The SP1TBI system provides unsurpassed defect sensitivity on bare wafers or unpatterned wafers; however, this is not the case when it is used for inspecting wafers with patterns thereon such as wafers with memory arrays, or for inspecting surfaces with much back ground noise.
Where rotational motion is caused between the illumination beam and the surface that is being inspected, it may be difficult to perform what is known as die-to-die comparison between pixel intensities of two different areas on the same surface that is being inspected. This is due to the fact that the angle of illumination of the two areas may be different, since the areas may be illuminated at different azimuthal angles, and the collection angles (both azimuthal and elevation) of scattered radiation may also differ between the two areas, depending on the timing of the rotation. If the two areas contain pattern, the pattern then may be at different orientations relative to the illumination beam and the collection optics so that a subtraction of the pixel intensities of the two areas or patches does not normally reduce noise caused by scattering due to pattern.
It is therefore desirable to provide a surface inspection system with capabilities that are better than those outlined above.